1. Field of the Invention
The present invention relates generally to television tuning systems and more particularly to such systems which operate to "seek" broadcast television signals having corresponding FCC allocated channel numbers.
2. Description of the Prior Art
A problem which is indigenous to all-channel television tuning systems is that the FCC television channel frequencies are allocated in four disconnected frequency bands. whereas the corresponding channel identification numbers (exclusively used by viewers for identification) run serially, without regard to the frequency band interruptions. In any given locale the allocated television channels may be widely dispersed throughout the VHF and UHF frequency bands, thus burdening the viewer with becoming familiar with an irregular sequence of non-related channel numbers for ordinary viewing.
Many signal seeking tuning systems have been proposed in the past, primarily to avoid the channel number problem and the inconvenience of tuning through non-operating channels or channel positions. No such system has, however, been successfully commercialized. In many of the proposed systems a ramp voltage generator, under control of a signal detector, supplies a voltage controlled oscillator for scanning of the appropriate frequencies. The received signal is usually filtered and a frequency discriminator is employed to produce a control potential (error signal) whose magnitude and polarity are the analog of the detected difference between the proper IF frequency and the actual frequency translated by the IF portion of the receiver. The error signal is used to correct the voltage of the ramp to adjust the tuner in the proper direction to "lock" to the signal. A threshold circuit may be used to skip over signals that are too weak for satisfactory viewing. These systems are confronted with the conflicting requirements of a "capture" range broad enough to pick up signals during sweeping and narrow enough to lock to proper signals only. This imposes significant limits on the sweeping speed.
In addition, in a signal seeking system the combined effects of capture range, lock range and the heterodyne process make is possible for an unwanted signal to "beat" with the local oscillator signal to produce a signal in the IF amplifier of proper frequency. These spurious or unwanted signals may be in the form of noise, image frequencies, or adjacent channel carriers. Many elaborate detection arrangements have been proposed to cope with the spurious signals in signal seeking tuning systems. Most require responses to more than one signal characteristic and there are some in the patent literature which detect as many as four distinct signal characteristics in an effort to preclude recognition of an erroneous signal.
The above problems were solved by the invention in copending related application Ser. No. 430,446 and U.S. Pat. No. 3,946,319. In the latter, a digital tuning system includes a counter which automatically sequences through channel numbers to provide tuning information for a varactor tuner to sweep through the broadcast television spectrum. It operates by bringing the tuner to a restricted frequency range about the picture carrier of the selected channel before enabling signal recognition apparatus to determine if a television signal is present. If not, the counter generates the next sequential channel number and the process is repeated.
The signal seeking system of the latter application also disposes illegal channel number detection means for automatically stepping the counter past non-allocated FCC channel numbers. The combination of elements producing a television signal seeking system for sequentially tuning to distinct ranges of frequencies corresponding only to FCC allocated channel numbers comprises the instant inventive system. Not only does this inactive system cover the spectrum more quickly, but more importantly, it materially aids the system noise immunity by precluding tuning to frequency ranges where there can be no proper television signals.